Scientists say millions of people face starvation following an outbreak of a deadly new strain of crop disease which is spreading across the wheat fields of Africa and Asia.

The disease, known as black stem rust, has already destroyed harvests in Uganda, Kenya and Ethiopia. Now researchers report that stem rust spores have blown across the Red Sea into the Arabian peninsula and infected wheat fields in Yemen. Spores have also blown northwards into Sudan.

Experts believe the disease - Puccinia graminis - will spread to Egypt, Turkey, the Middle East and finally India and Pakistan, which would lead to the destruction of the principal source of food for more than a billion people. Some observers warn that the disease could reach Egypt, which is heavily dependent on wheat, before the end of this year.

'This thing has immense potential for social and human destruction,' the international agriculture expert and Nobel prize-winner Norman Borlaug warned this month.

Black stem rust has blighted wheat production in many parts of the world for thousands of years. So pernicious were its effects that the Romans prayed to a stem rust god called Robigus.

'The Bible talks about plagues afflicting crops and these are almost certainly references to stem rust,' said Rick Ward, of the Global Rust Initiative, which has been set up to counter the new threat. When an outbreak occurred, a field of ripening wheat would be transformed into a mass of blackened vegetation.

Every few years one of these outbreaks would lay waste to entire harvests, sometimes sweeping across continents in only a few months.

But in the 1960s scientists and agriculture experts began to develop ways to counter the menace. Disease-resistant varieties of wheat were produced and planted in the West and in developing countries. As a result, it disappeared from most farms.

'Stem rust was something we felt we had solved,' Mariam Kinyua, a plant breeder at the Kenya Agricultural Research Institute in Njoro, told the journal Science this month.

However, a new strain of the fungus - known as Ug99 - was found in breeding nurseries in Uganda several years ago. The discovery caused alarm because virtually every variety of wheat tested with the strain was severely affected.

'Varieties that had been resistant for many years were no longer resistant,' said Wafa Khoury, a plant pathologist at the United Nations Food and Agriculture Organisation in Rome.

Within a year Ug99 was found in fields in Kenya and Uganda. The damage inflicted was severe but did not cause huge social problems because wheat is not a staple crop in either country. Nevertheless a field centre for Ug99 was established in Njoro and samples of wheat from around the world, including Argentina, Canada and Australia, were sent for testing. Virtually every single sample was found to be susceptible. 'That's what really caused the panic,' said Khoury.

This point was backed by Ward. 'The world had been safe for 50 years, but now the biblical plague that used to afflict our crops has returned. This is a very serious situation.'

The chilling feature about the new stem rust strain is the manner of its attack: Ug99 specifically targets resistance genes in wheat. As a result, it is now believed that 80 per cent of wheat varieties grown in the developing world are likely to be susceptible to the fungus. 'It's heaven for the [Ug99] pathogens,' said Khoury.

The spread of Ug99 to Yemen, where wheat is a staple, was confirmed in February by a team that included Khoury.

Now studies of wind patterns suggest Ug99 will soon spread to Saudi Arabia and the Near East. Eventually Syria, Lebanon, Turkey, Iran, Pakistan and Europe could be affected.

'We have to breed new wheat strains that are resistant to Ug99,' said Ward. 'If we do not, then we face the prospect of countries like Egypt and Pakistan suffering calamitous losses of wheat production. That would trigger all sorts of destabilising effects, ones that could have profound implications for the West. We have to move quickly. There is no time to lose.'